1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor thin film, and more particularly, to a method of forming a selective silicon epitaxial film.
2. Description of the Related Art
Accompanying the advances in the integration level of the semiconductor devices, refinement of the device dimensions has been in progress. In the refinement of the insulated gate field effect transistors (MOS transistors), it has been recognized that the short channel effect is a matter of concern. As a method of suppressing the short channel effect, a technique of reducing the depth of the source/drain diffused layer of the transistor has been conceived. Since, however, the method of mere reduction of the depth of the diffused layer brings about such problems as an increase in the sheet resistivity and an increase in the resistance at the contact part with the wiring material, there is proposed a method by which the region of formation of the source/drain diffused layer is elevated with a selective silicon epitaxial film. According to this method, it is possible to achieve simultaneously the formation of a shallow diffused layer and a reduction of resistance.
Now, in conventional selective epitaxy, regions of small film thickness where crystal planes with certain orientations, called facets, tend to be formed in the end parts (parts where the grown film makes contact with an insulating film) of the grown film. When these facets are formed, the depth of a diffused layer under such a facet region becomes large, making it difficult to form a shallow junction.
The cause of the formation of a facet is the difference in the surface energy which depends on the orientation of the crystal plane. The surface energy of silicon differs with the orientation of the crystal plane, for example, the (100) plane has a surface energy higher than those of the (111) plane and the (311) plane. Accordingly, on the silicon substrate of the (100) plane, epitaxial growth proceeds with the formation not only of the (100) plane but also of the (111) and (311) planes that have lower surface energies.
With this in mind, a method of growing a selective silicon epitaxial film without accompanying formation of facets is disclosed in Publication of Unexamined Patent Applications No. Hei 4-074415 and others.
According to Publication of Unexamined Patent Application No. Hei 4-074415, it is possible to form a facet-free selective silicon epitaxial film under an high vacuum of less than 10.sup.-3 Torr by lowering the growth temperature and increasing the supply amounts of the raw material gas for film formation. The ranges of the growth temperature and the flow rate of the raw material gas is such that at a growth temperature of below 550.degree. C., the flow rate of disilane (Si.sub.2 H.sub.6) is more than about 5 sccm. At 700.degree. C., it is more than about 50 sccm.
In addition, another method is known by which a desired silicon epitaxial film can be formed under a high vacuum of 1.5.times.10.sup.-4 Torr. Namely, in this method, a selective silicon epitaxial film almost free from facets can be formed even at a growth temperature as high as 700.degree. C. with a Si.sub.2 H.sub.6 flow rate of 1 sccm, by introducing atomic hydrogen obtained by letting hydrogen (H.sub.2) gas pass by filaments heated to a high temperature, along with the raw material gas Si.sub.2 H.sub.6 for film formation. In this case, a reaction chamber capable of maintaining ultra-high vacuum is required in order to produce and utilize atomic hydrogen.
However, according to the method in Publication of Unexamined Patent Application No. Hei 4-074415, for growth at a low temperature or high flow rate of Si.sub.2 H.sub.6, the film thickness for which the growth selectivity can be maintained becomes small so that it is difficult to apply the method to the manufacture of the actual device.
In the method of using atomic hydrogen, while it is possible to increase the thickness of the selectively grown film, the pressure during the growth has to be set at a high vacuum of less than about 1.5.times.10.sup.-4 Torr. Therefore, a CVD system for ultra-high vacuum (UHV-CVD system) with reachable degree of vacuum in the growth chamber on the order of 10.sup.-9 Torr is employed. The necessity for the use of an ultrahigh vacuum for the growth system applies also to Publication of Unexamined Patent Application No. Hei 4-074415.
The UHV-CVD system requires a constitution which minimizes the leakage in order to maintain its ultrahigh vacuum, and is very expensive compared with the low pressure (LP) CVD system which is in wide general use. Moreover, the work required for maintaining the ultrahigh vacuum is also more intricate compared with the case in the LPCVD system, and its productivity is deteriorated accordingly.
In view of the productivity and maintainability, selective silicon epitaxial growth by means of an LPCVD system rather than by a UHV-CVD system is desirable.
Now, in the selective growth by an LPCVD system, hydrogen chloride (HCl), in addition to dichlorosilane (SiH.sub.2 Cl.sub.2), the raw material gas for film formation, is used in order to obtain the required selectivity. Because of this, a tendency of generating changes in the growth rate depending on the size of the growth region and its density, the so-called loading effect, is conspicuous. The loading effect manifests itself when the growth pressure is high, so it is of no concern in the UHV-CVD system. In the LPCVD system on the other hand, formation of facets becomes a major problem when the growth pressure is lowered. Thus, in conventional LPCVD, the conditions which make selective silicon epitaxial growth possible through suppression of both of the loading effect and the facet formation, is extremely narrow compared with the case of growth in the UHV-CVD system, and it is not applicable to a stabilized mass production.